Comparison of Superconductivity and Structure for Lanthanum Replacing for Barium and Copper in YBa2Cu3Od

X-ray diffraction studies and the resistivity measurements are used to characterize the structure and the superconductivity of the nominal composition of YBa₂Cu_{3 – x} La _x O _d (YBCLO) cuprates with x 0.30. There was a BaCuO₂ impurity phase detected with x 0.10. The structure of the main phase (123) has the orthorhombic form with Pmmm symmetry in the whole doping range. With increasing content of lanthanum, x, the lattice constants increase for x < 0.04, and decrease for x 0.04. Rietveld refinements for X-ray diffraction show that the dopant of lanthanum substitutes for copper in the lower doping level, and replaces for both barium and copper in the high doping level. The zero-resistance temperature T _c0 first increases with the increase of the content of lanthanum in YBCLO as x 0.04 and then decreases with x as x 0.04. We compared the results with those of La-doped YBa_{2 – z} La _z Cu₃O _y cuprates. The different relationship in superconductivity dependence of lanthanum content may result from the strains due to the different occupancy of lanthanum in the unit cell of YBa₂Cu₃O _d .     

A realistic theory for the “normal state” and pairing mechanism in the high-tc cuprates

The electronic structure of the high-T _c cuprates is studied on the basis of large-U and smallU orbitals. Two types of charge carriers are predicted: (i) spinless polarons with a very small bandwidth; (ii) anomalous carriers of both charge and spin. The results are consistent with many anomalous properties of the cuprates. The spinless polarons undergo a CDW transition, and transitions between pair states of the two types of carriers provide the pairing mechanism.The author acknowledges stimulating discussions with participants of the workshop. Some numerical work was performed at the Florida State University Computing Center.     

Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7?y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1–x Zn _x )₃O_7–y withy0.1 has been measured forx0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7–y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx 0.10. YBa₂(Cu_1–x Zn _x )₃O_7–y retains the YBa₂Cu₃O_7-y orthorhombic structure forx0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,(T), is metallic-like (d/dT>0) and increases gradually with increasing Zn content. However, forx 0.10,(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Low-Field microwave measurements of YBa2Cu3O7?x near the superconducting transition temperature

Novel microwave absorption and dispersion measurements have been performed on well-characterized single-crystal platelets of the high-T _c superconductor YBa₂Cu₃O_7–x . The results are explained in terms of the rapid variation of the penetration depth near and belowT _c . Since EPR measurements are very sensitive to small changes in absorption and dispersion, this technique should be very useful in the understanding of the transition temperature region in both new and old superconducting materials.     

Magnetic Phase Diagram of Ca1?xMnxO

Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca_1–x Mn _x O. On increasing the Mn content, magnetic ordering changes from spin glass behavior for 0.25 x 0.4 to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for 0.5 x 0.65 and of first order for x 0.7. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.     

Room-temperature oxidation of La2?xSrxCuO4

A series of oxidized La_2–x Sr _x CuO_4+y compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

The Optical Conductivity of La2?xSrxCuO4 in Connection with a Charge Stripe Scenario

The in-plane optical conductivity of La_2–xSr_xCuO₄ with 0.07 x 0.19 exhibits strong peaks in the far infrared. These features, which depend both on doping and temperature, can be associated with excitations of charge stripes, basing on results of different techniques on the same system and on recent theoretical calculations.     

Anomalous Reentrant Resistance Phenomena in Superconducting Sr1 ? xKxBiO3: Recovery of Superconductivity with Electric or Magnetic Field

The temperature-dependent resistivity of Sr_{1 – x}K_xBiO₃, with x = 0.4–0.6, has been measured as a function of the magnetic field (or electrical current). Although X-ray diffraction results are more or less identical to single phase for the measured 10 samples, the EDS results indicate that the potassium content, x, varies from 0.4 to 0.6 and the electrical resistance varies quite sensitively from sample to sample. For the samples of resistivity less than 4 m cm at room temperature with (15 K)/(273 K) < 0.9, superconductivity is observed with T_c 12 K. Other samples exhibit a reentrant resistance below the superconducting transition temperature. The reentrant resistance, however, decreases as the external magnetic field (or electrical current) is applied, and some samples show the recovery of superconductivity upon the application of a magnetic field (or electrical current). Disorderness in the junction area between superconducting grains seems to be vital for the observed anomalous reentrant resistance. Further investigations are on to understand this intriguing phenomena.     

Electron–Phonon Interaction and Optical Spectra of Metals

Observed optical reflectivity in the infrared spectral region is compared with theoretical predictions in a strongly coupled electron–phonon system. Starting from a Fröhlich Hamiltonian, the spectral functions and their temperature dependence are derived. A full analysis including vertex corrections leads to an expression for the optical conductivity () that can be formulated in terms of the well-known optical conductivity for a quasi-isotropic system without vertex corrections. A numerical comparison between the full result and the so-called extended Drude formula, its weak coupling expansion, shows little difference over a wide range of coupling constants. Normal-state optical spectra for the high-T _c superconductors YBa₂Cu₃O₇ and La_{2 – x} Sr _x CuO₄ at optimal doping are compared with the results of model calculations. Taking the plasma frequency and from band structure calculations, the model has only one free parameter, the electron–phonon coupling constant . In both materials the overall behavior of the reflectivity can be well accounted for over a wide frequency range. Systematic differences exist only in the mid-infrared region. They become more pronounced with increasing frequency, which indicates that a detailed model for the optical response should include temperature-dependent mid-infrared bands.     

Microscopic Scenario for x = 1/8 Anomaly in La2 ? xSrxCuO4

We adopt a t ₁-t ₂-t ₃-J-G model for explanation of x = 1/8 anomaly in La_{2 – x} Sr _x CuO₄ family compound. The calculated charge susceptibility shows a maximum near Q = (, ) at intermediate temperatures and near (, /2) as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion G between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band t ^eff ₁, t ^eff ₂, t ^eff ₃ < G. For physically realistic hopping values we obtain the CDW amplitude e _Q = x. The in-phase domain structure as a candidate for stripe picture is proposed.     

Optical Properties, Electronic Structure and Magnetism of α′-NaxV2O5

The optical properties of sodium-deficient -Na _x V₂O₅ (0.85 x 1.00) single crystals are analyzed using ellipsometry, and infrared reflectivity techniques. In sodium deficient samples, the optical absorption peak associated to the fundamental electronic gap develops in the middle of the pure -NaV₂O₅ gap at 0.44 eV, and the material remains insulating up to the maximal achieved hole concentration of about 15%. Nonmetallic behavior under hole doping provoked reinterpretation of the -NaV₂O₅ optical spectra. We argue that the absorption peak at about 0.9 eV corresponds to the photoionization energy of a large polaron.     

Bulk thermal expansion studies of BiCaSrCu2O x and Bi2CaSr2Cu2O x

The compounds BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x were prepared by ceramic techniques and characterized by X-ray powder diffractometry (XRD) and microthermogravimetry (TG) and their bulk thermal expansion measurements were carried out using dilatometry in the temperature range 298T1073 K in air. The results have been analyzed and are compared with those obtained earlier for YBa₂Cu₃O₇. The XRD analysis shows that both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x are single phase in nature, having an orthorhombic symmetry. The TG analysis carried out in oxygen, air, and nitrogen shows negligible weight loss (R~0.1%) on heating to 1073 K, indicating that these two compounds, unlike YBa₂Cu₃O₇, are quite stable. The analysis of bulk thermal expansion data reveals that the average linear thermal expansion coefficient ( ₁) for both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is almost the same ( ₁ 10.5×10^–6 K^–1) and is found to be nearly half of that for YBa₂Cu₃O₇ ( ₁ 18×10^–6 K^–1), suggesting that the interatomic bonding in both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is stronger as compared to YBa₂Cu₃O₇.     

Thermoelectric Power Studies on YBa2?xCaxCu3O7?δ System

The temperature dependence of the thermoelectric power of calcium substituted YBa_2–x Ca _x Cu₃O_7– pellets with 0x1.5 is presented between 60 K and 300 K. A metal–insulator transition was reported earlier by us in this system and was attributed to the ionic size-dependent localization effect. While the sign of thermoelectric power of all the calcium substituted samples was found to be positive, its magnitude increases significantly with calcium content in YBCO. The normal state thermoelectric power data of substituted YBa_2–x Ca _x Cu₃O_7– (0x1.5) are discussed in light of a two-band model originally proposed by Gottwick et al. for heavy fermion systems and later modified by Forro et al.     

Extraordinary behaviour of the Y1?xPrxBa2Cu3O7?δ system

Investigations of Y_1–x M _x Ba₂Cu₃O_7– (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT _c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals reveal aT ² dependence of the cotangent of the Hall angle in the normal state and a negative Hall anomaly belowT _c in the superconducting state, in agreement with recent reports. Our research shows that the depth, , of the negative Hall signal scales withT/T _c and that the maximum value of decreases linearly withx and vanishes atx0.24. Magnetoresistance measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals indicate that the irreversibility lineH(T ^*) obeys a universal scaling relation characterized by anm=3/2 power law nearT _c, with a crossover to a more rapid temperature dependence of belowT/T _c 0.6, similar to that observed for polycrystalline specimens.     

Polymorphism in Sr3Fe2O7−x

The compound Sr₃Fe₂O_7–x , with variable iron valence, was investigated by X-ray powder techniques, both at room and at high temperatures. If the material is examined in massive form, a single phase called -Sr₃Fe₂O_7–x appears as previously reported in the literature. This -phase is tetragonal and exhibits the lattice parameters: a=3.874 and c=40.314 Å. Two other phases, called and -Sr₃Fe₂O_7–x , respectively, can be obtained on heating the finely powdered material when laid on a flat platinum support. The form is stable up to 1275° C, while the form is revealed only above 1275° C and changes always into -Sr₃Fe₂O_7–x when quenched. Both and phases are tetragonal, with a=4.001 and c= 58.251 for the form and a=4.013, c=57.092 Å for the form. The transition involves a true phase equilibrium, while the transformation is possible only by means of a suitable mechanical treatment of the material.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 m, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.     

Single-crystal growth and anisotropic electrical properties of (La1?xCax)2CaCu2O6

Large single crystals of (La_1–x Ca _x )₂CaCu₂O₆ were synthesized by the travelling solvent floating zone (TSFZ) technique. The dimension of the grown boules was typically 4 mm×30 mm long with thec-axis perpendicular to the growth direction. After oxygen loading at 1080°C in 400 atm of O₂, they became superconductive with a sharp transition at around 50 K. The resistivity was metallic both along thec-axis and within theab-plane, with an anisotropy ratio of ²( _c/ _ab ) 50 which is almost independent of temperature. Details of the crystal growth, heat treatment procedures, and the results of electrical and magnetic property measurements are presented.     

Superconductivity of YBa2?xNdxCu3Oy Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2–x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x0.30. The onset transition temperature is insensitive to the Nd content x in the region of x0.40. All are higher than 95 K. The zero resistance transition temperatures , however, exhibits two-step variation with the increase of x. For x0.25, are all above 92 K. The highest of 94 K was obtained for x=0.25. For x0.3 drops sharply to about 84 K. Finally falls to 30 K and is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.